1. Field of the Invention
The present invention relates generally to a user selection apparatus and method for implementing Spatial Division Multiple Access (SDMA) in a Multiple Input Multiple Output (MIMO) system, and in particular, to an apparatus and method for selecting users with a reduced computation volume in a MIMO system supporting SDMA.
2. Description of the Related Art
With the recent advent of the wireless multimedia era, the rapid increasing demands for high-speed transmission of a large amount of data on radio channels have driven the worldwide development of wireless, high-speed data transmission systems to support Internet service on mobile channels and wireless channels.
For mobile, wireless Internet services, MIMO has attracted a lot of attention as a prominent scheme of increasing data rate.
The MIMO system, using a plurality of transmit and receive antennas, offers high data rates and outperforms a non-MIMO system in terms of the capacity of a radio link between a transmitter and a receiver. Under a multipath-rich environment, a plurality of orthogonal channels can be generated between the transmitter and the receiver. Hence, data for a single user can be sent by radio resources in parallel on the orthogonal channels with higher spectral efficiency, although using the same bandwidth as in the non-MIMO system.
Furthermore, since the MIMO system supports SDMA, Signal-to-Noise Ratio (SNR) is improved and higher data rates are available.
FIG. 1 is a flowchart illustrating a conventional operation for selecting SDMA users in a Base Station (BS) in a MIMO-SDMA system. The following description is based on the assumption that U SDMA users are selected from among K users using a single receive antenna (R=1), and the BS uses T antennas and has knowledge of channel information between the T antennas and the individual users. Notably, U is equal to or greater than 2 (U≧2).
Referring to FIG. 1, the BS generates all possible combinations of U users from K users in step 101.
In step 103, the BS calculates the capacity of a jth user combination by Equation (1), Equation (2), Equation (3) and Equation (4) set forth below. j is the index of the total KCU combinations and its initial value is 1.
A weight vector for the jth user combination is calculated using the channel information H of the U users in the jth user combination byWZF=HH(HHH)−1=[W1,W2, . . . , WU]  (1)where WZF denotes a zero-forcing weight vector and H denotes the total channel matrices [H1, H2, H3, . . . , HU]T of the U users each having R receive antennas. Hu is the channel matrices between the T transmit antennas of the BS and the R receive antennas of a Uth user, [H1U, H2U, H3U, . . . , HRU]T, where HRU denotes the channel matrix between the T transmit antennas of the BS and an Rth receive antenna of the Uth user, [HR,1U, HR,2U, HR,3U, . . . , HR,TU]T. HR,TU denotes the channel between a Tth transmit antenna of the BS and the Rth receive antenna of the Uth user.
Using the weight vector WZF, a power scaling factor of each user is calculated byPu=1/|Wu|2  (2)where Wu denotes a weight column vector [W1u, W2u, . . . , WTu]T applied to the T transmit antennas for a uth user.
The SNRs of the U users are calculated using their power scaling vectors. For the uth user in the jth user combination,
                              SNR          u                =                                                            P                u                            ⁢                                                                                      S                    u                                                                    2                                                    σ              u              2                                =                                                                                                            S                    u                                                                    2                            /                                                                                      W                    u                                                                    2                                                    σ              u              2                                                          (        3        )            where Pu denotes the power scaling factor of the uth user calculated by Equation (2), Su denotes a transmission symbol transmitted in SDMA to the uth user, and σ2u denotes the noise power of the uth user.
Using the SNRs of the users, the total capacity of the jth user combination, Cjtotal is given by
                              C          j          total                =                              ∑                          u              =              1                        U                    ⁢                                    log              2                        ⁡                          (                              1                +                                  SNR                  u                                            )                                                          (        4        )            
After calculating the total capacity of the jth user combination, the BS compares j with kCu to determine whether the capacity of every user combination has been calculated in step 105. If j is less than kCu (j<kCu), which implies that there still remain user combinations whose capacities are to be calculated, the BS increases j by 1 (j=j+1) in step 107 and returns to step 103.
On the other hand, if j is equal to or greater than kCu (j≧kCu), which implies that the capacity of every user combination has been calculated, the BS selects a user combination that maximizes capacity and transmits data to the users of the selected user combination in SDMA in step 109. Then the BS ends this algorithm.
As described above, the conventional MIMO system generates all possible user combinations from total users to select SMDA users, calculates the capacity of every user combination, and selects a user combination offering the maximum capacity. Therefore, a large volume of computation is required.